Problem: On the first 3 tests of her geometry class, Gabriela got an average score of 80. What does she need on the next test to have an overall average of 85?
Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 80 + x$ The same sum must also be equal to $4 \cdot 85$ Solve: $x = 4 \cdot 85 - 3 \cdot 80 = 100$.